This project addresses two types of eye movement (saccades, OKN), with emphasis on timing in response to visual stimuli. Saccades are rapid movements made when scanning a visual scene. Saccades to periodically-paced targets have a characteristic latency: as pacing rate increases saccades become more anticipatory. OKN (optokinetic nystagmus) occurs with wide-field motion of a scene, and is an indicator of vestibular and oculomotor function. During OKN, the eyes track the field, and intermittently reset their position by moving rapidly in the opposite direction. The eye position waveform looks like a fluctuating sawtooth. OKN is not periodic; models typically rely on statistical characterization of the variations, and recent work indicates that the dominance of random vs. deterministic dynamics makes this reasonable. If a subject actively follows the scene ("look nystagmus"), OKN is different from when a subject simply stares straight ahead ("stare nystagmus"). This may reflect different degrees of reflexive and volitional control. In the first sub-project, scaling properties of periodically-paced saccades will be studied. Many systems have a characteristic scaling behavior, such that variability increases as a power-law function of sequence length, reflecting a type of statistical long-term memory. Scaling of saccade latencies will be assessed with a parametric procedure, and verified graphically. The goal is to assess scaling over the range where saccades are reactive or predictive (latencies positive or negative, respectively). Evidence is that scaling behavior in the predictive regime is different from that in the non-predictive ("reactive") regime. This relationship can link reflexive/volitional behaviors to predictive/reactive behaviors. Evidence is also presented for altered scaling in cerebellar patients. The second sub-project will apply similar analyses to OKN fast-phase intervals. Evidence indicates that scaling is altered depending on the degree of volitional vs. reflexive behavior. This volitional/reflexive mix can be altered through stimulation of different types of OKN (horizontal vs. torsional, "look" vs. "stare"), and used to verify the relationship of scaling to behavior. The results will be widely applicable to physiologic systems that can be described by statistical scaling laws, and to systems which exhibit combinations of reflexive and volitional behavior and prediction. Since scaling changes with pathology, new means to assess vestibular or oculomotor pathology may result.